Here are three fitness apps for Kindle:
My Yoga Studio (A Yoga Partner on Kindle) - $1.99
Anywhere Abs (A Workout Partner on Kindle) - $1.99
Anywhere Legs (A Workout Partner on Kindle) - $1.99
I haven't tried any of them myself. The first two have very good overall reviews at Amazon. The last one was just released this week.
Active Content, such as these, is only available in the U.S. at this time. It's very interesting to see what the app developers are coming up with!